The field of the invention is nuclear magnetic resonance imaging (xe2x80x9cMRIxe2x80x9d) methods and systems. More particularly, the invention relates to the acquisition of partial velocity encoded MRI data sets and the reconstruction of images from such data sets.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
The present invention will be described in detail with reference to a variant of the well-known Fourier transform (FT) imaging technique, which is frequently referred to as xe2x80x9cspin-warpxe2x80x9d. The spin-warp technique is discussed in an article entitled xe2x80x9cSpin-Warp NMR Imaging and Applications to Human Whole-Body Imagingxe2x80x9d by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gy) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gy is incremented (xcex94Gy) in the sequence of views that are acquired during the scan to sample so-called xe2x80x9ck-spacexe2x80x9d and thereby produce a set of NMR data from which an image can be reconstructed. The phase encoding gradient Gy steps from a negative value through zero to a corresponding positive value to sample k-space symmetrically around its origin.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. The reduction of scan time in ECG gated cardiac imaging is particularly important in order to acquire the image data within a single patient breath-hold. This avoids respiratory gating and the introduction of image artifacts caused by respiratory motion.
One method for reducing scan time is to reduce the total number of views acquired during the scan. Instead of sampling k-space symmetrically around the origin, only spatial frequencies on one side of the origin plus a small amount near the origin on the opposite side are sampled. For example, instead of stepping Gy through 128 values ranging from xe2x88x9264 to +64, only the views ranging from xe2x88x9264 to +8 are acquired. As a result, fewer views are acquired which shortens scan time, but some k-space data is missing from the acquired data set.
Another method for reducing scan time is to acquire a partial NMR echo signal as described in U.S. Pat. No. 5,168,227. This moves the time to the echo signal peak (TE) closer to the start of the readout gradient waveform, shortens the readout gradient waveform, and shortens the transmit repeat time (TR) of the pulse sequence. For example, the TR of a gradient recalled echo pulse sequence can be shortened from 7-8 msec. to 2-6 msec. with a resulting 20% reduction in scan time to acquire an image. However, k-space along the readout gradient axis is not fully sampled when a partial echo is acquired and some k-space data is missing in the resulting k-space image data set.
There are two basic methods used to reconstruct images from such xe2x80x9cpartialxe2x80x9d Fourier image data sets. The first method is referred to in the art as xe2x80x9czero fillingxe2x80x9d. As the name suggests, the missing k-space data is set to zero and a normal Fourier transformation of the zero-filled k-space image data set is performed to reconstruct an image. Unfortunately, the magnitude image produced with a zero-filled k-space data set has reduced resolution, or spatial blurring.
A second method for reconstructing an image from a partial Fourier image data set uses Hermitian conjugate symmetry to replace the missing k-space data. Hermitian conjugate symmetry only works if the image is real. Numerous phase errors are present in MRI data that make the image complex. These phase errors result from phenomena such as B0 inhomogeneity, gradient eddy currents, group delays in the gradient amplifiers and receive electronics, and the spatial variation of surface coil receive B1 fields. To enable Hermitian conjugate replacement to work with a complex image, the replacement of the missing k-space data is accompanied by a phase correction which removes the phase errors from this data. One partial Fourier reconstruction algorithm, called xe2x80x9cHomodyne reconstructionxe2x80x9d, uses two filters to accomplish the Hermitian conjugate replacement and the phase correction, respectively, Noll D C, Nishimura O G, and Macovski A, xe2x80x9cHomodyne Detection in Magnetic Resonance Imaging,xe2x80x9d IEEE Trans. Med. Imaging 991; 10:154-63. A Homodyne high pass filter doubles the amplitude of the acquired k-space data which is conjugate to the missing k-space data prior to the Fourier transform. After the Fourier transform and phase correction, the imaginary part of the image is discarded to complete the replacement step. The phase correction step is accomplished by a Homodyne low pass filter. This filter creates an image from a small portion of k-space data acquired symmetrically around the center of k-space. The phase of this image is subtracted from the phase of the Homodyne high pass filtered image prior to discarding the imaginary part of the image.
Although some phase information may be preserved when performing the Homodyne reconstruction, it has been found that phase images produced with this method have increased spatial blurring.
MR images which indicate the velocity of moving spins may be acquired by employing a velocity encoding gradient in the pulse sequence as described in U.S. Pat. No. 5,093,620. Such images indicate blood flow velocity in images and they are useful to provide quantitative measurements of blood flow volume through arteries. To reconstruct a velocity image, however, the phase information in the acquired k-space image data set must be accurately preserved. As a result, MR velocity imaging has necessitated the acquisition of the entire Fourier image data set.
The present invention is a method and apparatus for reconstructing both magnitude and phase images from partial Fourier MR image data sets. More specifically, a complex partial k-space image data set is acquired using a pulse sequence which employs a velocity encoding gradient, a first complex image is reconstructed from the complex partial k-space image data set using a homodyne reconstruction process, a magnitude image is constructed from the real part of the first complex image, the complex partial k-space image data set is zero-filled, a second complex image is reconstructed from the zero-filled complex k-space image data set, and a velocity image is constructed from phase information in the second complex image. The magnitude image may be used to obtain an accurate measurement of an artery or other structure through which blood is flowing and the velocity image may be used to measure the average velocity. From these measurements, blood flow volume may be computed.
The present invention has particular application where blood flow through an artery, heart valve, or other structure is to be measured and motion due to patient respiration is an issue. A partial Fourier acquisition may be employed to shorten scan time and enable the data to be acquired in a single breath-hold. Accurate anatomic information is produced by the magnitude image and accurate velocity information is produced from the velocity image. An accurate depiction of the vessel boundaries is important as the flow (i.e., velocity times cross-sectional area) is highly dependent on the correct measure of the anatomic cross-sectional area of the vessel.